1. Field of the Invention
This invention relates to a synthetic fiber, in particular a PVA fiber, showing good reinforcing property when added to a hydraulic substance such as cement. It also relates to a material of construction as obtained by incorporating said synthetic fiber into a hydraulic substance and having good workability and reinforcing property.
2. Description of the Prior Art
Moldings made of a hydraulic substance such as cement mortar or concrete are brittle bodies and are therefore disadvantageous in that they have very low flexibilitiy, that they are readily broken or cracked when a tensile or bending force is applied thereto, and that they are poor in shock resistance. To remove these drawbacks, steel reinforcements or skeletons are combinedly used. It is also known to reinforce such moldings by addition of various kinds of fiber.
Previously, the present inventors proposed a technique of remedying such drawbacks of cement and concrete (hereinafter referred to "mortar and the like" for short) as described in Japanese Kokai Tokkyo Koho (Published Unexamined Patent Application) No. 8664/84. Subsequent investigations, however, revealed that the 100-1,000 denier (Dr) synthetic fiber monofilaments to be used in accordance with the technique disclosed in said Japanese Kokai Tokkyo Koho No. 8664/84 indeed show good reinforcing property but are disadvantageous in that they have poor dispersibility and cause decrease in the flowability of mortar and the like, hence they offer workability problems. As a result of various investigations to cope with the problems, it was concluded that a fineness of not less than 1,000 Dr is essential. In the art, however, no technique was available for the production of synthetic fiber monofilaments having a fineness of not less than 1,000 Dr and a high tenacity necessary for reinforcing purposes.
As a synthetic fiber monofilament species having a fineness of not less than 1,000 Dr, a polyethylene fiber (BONFIX.RTM.; 0.9 mm in mean diameter) is commercially avilable. However, the reinforcing effect of this fiber is unsatisfactory because of its low strength, although the workability and other features of the fiber are good. The present inventors, too, made intensive investigations in an attempt to develop a technique of producing high tenacity monofilaments having a fineness of not less than 1,000 Dr using polyvinyl alcohol (hereinafter referred to as "PVA"), but in vain.
Owing to these circumstances, the present inventors attempted to improve the state of art by bundling fine-denier filaments which can be bestowed with high tenacity rather easily. Thus, as proposed in Japanese Kokai Tokkyo Koho No. 21559/85, a technique was provided by the present inventors which comprises twisting multifilaments each composed of high-tenacity, finedenier filaments, binding the fine-denier filaments with a hydrophobic resin, and incorporating the thusobtained fiber bundle yarns having a fineness of up to 5,000 Dr into mortar and the like. Since said fiber bundle yarns will not be separated into single fibers even in the step of kneading with mortar and the like, good dispersibility can be attained and the workability problems can be solved. However, coating of the surface of a PVA fiber, which is by nature good in adhesion to mortar and the like, with a hydrophobic resin results in decreased adhesion to mortar and the like and, as a result, the reinforcing property of the fiber bundle yarns is unsatisfactory in spite of the fact that said yarns themselves have high tenacity. In addition, twisting of multifilaments, treating them with a binding agent, drying them and curing them are required for the production of said fiber bundle yarns. The number of production steps is thus increased and, moreover, the productivity is low, hence the production cost is increased. This is a particularly great obstacle from the practicability viewpoint.
As mentioned above, no fiber has been found to be satisfactory as a reinforcing fiber for mortar and the like when evaluated in terms of reinforcing property and workability. Appearance of a satisfactory reinforcing fiber for mortar and the like has earnestly been waited for in the art.